Regarding a rotational angle sensor, as disclosed in JP 2012-145488 A (Patent Document 1), a technique for diagnosing a failure in the rotational angle sensor based on whether or not a sum of a square of a sine wave signal (sin θ) and a square of a cosine wave signal (cos θ) corresponding to a rotational angle θ of a rotating body (i.e., sin2 θ+cos2 θ) is within a normal range, has been proposed.